1. Technical Field
The present invention relates generally to the field of spectral photoluminescence mapping on transparent films, such as these found in Light Emitting Diodes (LED). The present invention may also be applicable to spectral electroluminescence mapping on transparent films. The substrate the films are deposited on may be transparent or opaque.
2. Description of the Background Art
Photoluminescence (PL) and electroluminescence (EL) are spectroscopy techniques providing information on electrical and optical properties of semiconductor materials, such as bandgap, emission wavelength, composition, defects, and so on. A PL mapper is a laser-based instrument used to generate parameter maps over a wafer by measuring optical luminescence emission from materials excited with energy above their bandgap. An EL mapper excites excess carriers using direct current injection, rather than a laser source. An EL mapper is typically used on final devices where electrical contact pads already exist, but may also be used on an epi layer after the MOCVD deposition step. The value of the EL measurement is that it adds electrical properties to the luminescence measurement, such as the forward voltage, the reverse current, or the I(V) curve of the junction that will constitute the heart of the finished LED.
A typical light emitting diode (LED) wafer after metal organic chemical vapor deposition (MOCVD) of the active layers includes the substrate (typically a sapphire wafer from two to six inches in diameter), GaN buffer layers (to help accommodate the lattice mismatch between the sapphire and the critical layers), a negatively doped (n-GaN) contact layer, a multi-quantum well (MQW) multilayer hetero-structure, an optional electron blocking layer and a positively doped (p-GaN) contact layer.
Metrology tools that sample an entire area of the LED epitaxial (epi) layer post-MOCVD may be referred to as a photoluminescence (PL) mapper. The operating principle of these tools involves exciting the MQW at shorter wavelength than the emission wavelength hereby generating electron-hole pairs in the active region and detecting the spectral emission line from the radiative recombination of electrons and holes as a function of the location on the wafer. PL mappers may report important statistics describing the local luminescence spectra, such as: peak wavelength, peak intensity and full-width at half-maximum (FWHM) of the local emission spectrum observed. The typical spatial resolution for such mapping is on the order of one millimeter, which is generally sufficient for the typical spatial variations of the emission wavelength, as these variations are mainly coming from MOCVD temperature gradients which occur at millimeter and longer length scales. The “peak wavelength” is typically measured over the whole area of all the epi wafers from an MOCVD deposition run, and used by process control engineers to directly flag the temperature gradients that existed in the MOCVD process for that wafer: The typical wavelength shift expected is on the order of approximately 2 nanometers per degree Kelvin (nm/K), and it thus only takes a deposition temperature gradient on the order of one degree to affect the bin yield for the final devices. Accordingly, very tight temperature drift and gradient control are mandated at the elevated growth temperature found in the reactor, and a systematic control for each outgoing wafer is needed as its individual temperature uniformity is also a function of how well that wafer sat in the MOCVD carrier pocket. A large range of peak wavelength variations within a given wafer may be indicative that the MOCVD reactor's wafer carrier needs to be cleaned or replaced. The FWHM and peak intensity (brightness) are also important and may be related to the quality and uniformity of the epi layer and MQW composition and structure.